Blogging can be one of the best jobs in the world if you're good at it - you can earn as much or as little money as you like, and you get to write about something that really interests you and that you are passionate about. If you have areas that encourage visitor interaction, such as forums or comments, then you can really have a good laugh.
Firstly you need to come up with a good idea to base your blog on - it needs to be something that you really like and find interesting so that you will want to write on it very regularly. You can either choose a very niche market to aim for as this will get a lot of expert traffic and you will get good decisions from advertisers, or you can aim for the mass market and get lots of visitors who will make you a little bit of money from each advert All you need to do is start blogging, and market it by linking from other websites and submitting to directories and search engines.
When you have enough visitors and posts you can start making money - many people used to use Google AdSense for adverts on their website, but this has lost popularity and people now go for affiliate networks to make money. Affiliate networks are where your visitors sign up for a product or service that they may like and you get a percentage of the price. Easy! This is by far the best way of making money with blogging.
However, there are other ways of making money instead of using adverts or affiliate websites. You can sell merchandise relating to your site if your blog is popular enough - simply sign up with someone such as CafePress and you can sell your own products without even having to stock them!
Another way you could try is selling posts on your site. People will pay money to write a 'guest post' for your website with the promise of a link back to their website, as it will increase their exposure and search engine rankings. This is a good way to make money and at the same time providing variety to your readers!
You can also sell links from various pages of your website in order to generate revenue. Selling links will increase the search engine rankings of the website you're linking to, similar to guest posts as mentioned above.
You may wish to look for sponsorship for your blog - some companies will pay you money to say that you are sponsored by them and place links to their website and their adverts on your blog in return.
Hopefully that has given you some ideas on how to make money from your blog, with the most popular at the moment probably being affiliate marketing campaigns - try it out! Of course if you can not seem to make any money with the other techniques, you could always ask for donations from visitors!